Noncovalent Control of the Electrostatic Potential of Quantum Dots through the Formation of Interfacial Ion Pairs

Journal of the American Chemical Society

5 years ago

Noncovalent Control of the Electrostatic Potential of Quantum Dots through the Formation of Interfacial Ion Pairs

Monica Olvera de la Cruz, Trung D. Nguyen, Chen He, Kedy Edme, Emily A. Weiss

This paper describes the role of tetraalkylammonium counterions [NR₄⁺, R = −CH₃, −CH₂CH₃, −(CH₂)₂CH₃, or −(CH₂)₃CH₃] in gating the electrostatic potential at the interface between the 6-mercaptohexanoate (MHA) ligand shell of a PbS quantum dot (QD) and water. The permeability of this ligand shell to a negatively charged anthraquinone derivative (AQ), measured from the yield of electron transfer (eT) from the QD core to AQ, increases as the steric bulk of NR₄⁺ increases (for a given concentration of NR₄⁺). This result indicates that bulkier counterions screen repulsive interactions at the ligand/solvent interface more effectively than smaller counterions. Free energy scaling analysis and molecular dynamics simulations suggest that ion pairing between the ligand shell of the QD and NR₄⁺ results from a combination of electrostatic and van der Waals components, and that the van der Waals interaction promotes ion pairing with longer-chain counterions and more effective screening. This work provides molecular-level details that dictate a nanoparticle’s electrostatic potential and demonstrates the sensitivity of the yield of photoinduced charge transfer between a QD and a molecular probe to even low-affinity binding events at the QD/solvent interface.

Publisher URL: http://dx.doi.org/10.1021/jacs.7b05501

DOI: 10.1021/jacs.7b05501